feat(build): build.zig MVP: build and run functionaltests on linux

NEW BUILD SYSTEM!

This is a MVP implementation which supports building the "nvim" binary,
including cross-compilation for some targets.
As an example, you can build a aarch64-macos binary from
an x86-64-linux-gnu host, or vice versa

Add CI target for build.zig currently for functionaltests on linux
x86_64 only

Follow up items:

-  praxis for version and dependency bumping
-  windows 💀
-  full integration of libintl and gettext (or a desicion not to)
-  update help and API metadata files
-  installation into a $PREFIX
-  more tests and linters

cmake.config/versiondef.h.in

@@ -12,6 +12,6 @@
 #endif
 
 #define NVIM_VERSION_CFLAGS "${VERSION_STRING}"
-#define NVIM_VERSION_BUILD_TYPE "$<CONFIG>"
+#define NVIM_VERSION_BUILD_TYPE "${CONFIG}"
 
 #endif  // AUTO_VERSIONDEF_H